1. Field of the Invention
This invention relates to a pump, more particularly to a manually operable pump.
2. Description of the Related Art
The improvement of this invention is directed to a conventional manual pump which is disclosed in the applicant's U.S. Pat. No. 5,156,538, issued on Oct. 20, 1992, the disclosure of which is hereby incorporated by reference. The conventional manual pump comprises an elongated tubular member which has an upper closed end and a lower open end with a first opening. The elongated tubular member further has a second opening on its side wall and a piston movable therein. The elongated tubular member is disposed rotatably and engagably in a first tubular member. The first tubular member has first inflow and outflow check valves, second inflow and outflow check valves, and a third opening. When the elongated tubular member is rotated in the first tubular member, the first and second openings of the same can be selectively aligned with the corresponding first inflow or outflow check valve and with the corresponding third opening or second inflow or outflow check valves of the first tubular member. The elongated tubular has a circumferential projection which extends into a circumferential recess that is formed on the inner face of the first tubular member in order to preclude disengagement of the elongated tubular member from the first tubular member. The disadvantage of the conventional manual pump resides in that the manufacture of the mold which is used to form the first tubular member with the check valves and the circumferential recess is difficult to conduct, thereby resulting in a relatively high mold-manufacturing cost.